It has long been known that when an electric current flows in a wire it generates a magnetic field about that wire. The magnetomotive force generated by the current in the wire can be strong enough to reverse the magnetic polarity of a retentive magnetic material placed within the magnetic influence of that magnetomotive force. When the retentive material changes polarity, it induces a voltage pulse in the wire, as well as in a second sense wire if placed within the magnetic influence of the magnetic material. Such a voltage pulse can be used to indicate that the magnetic material has changed polarity. This is the technique used to interrogate the intelligence stored in toroidal ferrite cores used in the memories of digital computers.
When an external magnetomotive force field, as from a magnetic recording tape or the Earth's magnetic field is to be sensed, a magnetically permeable material, whether retentive or not, is brought within that magnetic field. The magnetic field can magnetically saturate the material or at least distort the symmetry of its response to the magnetomotive force generated by an alternating current in an associated drive wire, sometimes called an excitation winding. If the magnetic material saturates, a sense wire or winding associated therewith will note a sharp reduction in the transformer coupling that otherwise exists from the excitation winding, through the magnetic material, and to the sense winding. The excitation winding and the sense winding correspond to the primary and secondary windings, respectively, of a transformer.
When the external magnetic field only distorts the symmetry of the response of the magnetic material, the output voltage from the sense winding contains an increased proportion of even-numbered harmonics of the excitation alternating-current signal impressed on the excitation winding. Saturable reactors, magnetometers, and transformer-like structures of all types have long been known and used for magnetic field sensing.
U.S. Pat. No. 3,521,261 granted on July 21, 1970, to J. L. Metz is one example of such a magnetic sensor. The prior art is filled with physical arrangements of a magnetic material and its primary and secondary windings. Two examples of such prior art are shown in U.S. Pat. Nos. 2,916,696 granted on Dec. 8, 1959, and 3,439,264 granted on Apr. 15, 1969, both to E. O. Schoenstedt. These two patents show a very thin "Permalloy" strip wrapped about one or more conductors and in turn surrounded by a sense winding.
The prior-art magnetic sensing devices are limited since they use intrinsic, quantitative characteristics of the magnetic material, such as coercive force or saturation flux density, to sense the presence or magnitude of a magnetic field. Therefore, these prior-art sensing devices lack absolute accuracy and linearity, in the quantitative sense, in the measurement of a magnetic field or the electric current that develops it.